1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method. This invention also relates to a device manufactured thereby.
2. Background of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Between the reticle and the substrate is disposed a projection system for imaging the irradiated portion of the reticle onto the target portion of the substrate. The projection system includes components for directing, shaping or controlling the projection beam of irradiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example.
Generally, the projection system comprises structure to set the numerical aperture (commonly referred to as the “NA”) of the projection system. For example, an adjustable NA-diaphragm is provided in a pupil of the projection system. The illumination system typically comprises adjustable optical elements for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution upstream of the mask (in a pupil of the illumination system). A specific setting of σ-outer and σ-inner may be referred to hereinafter as an annular illumination mode. Controlling the spatial intensity distribution at a pupil plane of the illumination system can be done to improve the processing parameters when an image of the illuminated object is projected onto a substrate.
Microchip fabrication involves the control of tolerances of a space or a width between devices and interconnecting lines, or between features, and/or between elements of a feature such as, for example, two edges of a feature. In particular the control of space tolerance of the smallest of such spaces permitted in the fabrication of the device or IC layer is of importance. Said smallest space and/or smallest width is referred to as the critical dimension (“CD”).
With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD for isolated features and dense features may limit the process latitude (i.e., the available depth of focus in combination with the allowed amount of residual error in the dose of exposure of irradiated target portions for a given tolerance on CD). This problem arises because features on the mask (also referred to as reticle) having the same nominal critical dimensions will print differently depending on their pitch on the mask (i.e., the separation between adjacent features) due to pitch dependent diffraction effects. For example, a feature consisting of a line having a particular line width when in isolation, i.e., having a large pitch, will print differently from the same feature having the same line width when together with other lines of the same line width in a dense arrangement on the mask, i.e., having small pitch. Hence, when both dense and isolated features of critical dimension are to be printed simultaneously, a pitch dependent variation of printed CD is observed. This phenomenon is called “iso-dense bias,” and is a particular problem in photolithographic techniques. Iso-dense bias is typically measured in nanometers and represents an important metric for practical characterization of lithography processes.
Conventional lithographic apparatus do not directly address the problem of iso-dense bias. Conventionally, it is the responsibility of the users of conventional lithographic apparatus to attempt to compensate for the iso-dense bias by either changing the apparatus optical parameters, such as the NA of the projection lens or the σ-outer and σ-inner settings, or by designing the mask in such a way that differences in dimensions of printed isolated and dense features are minimized.
Generally, in a high volume manufacturing site different lithographic projection apparatus are to be used for the same lithographic manufacturing process step to ensure optimal exploitation of the machines, and consequently (in view of, for example, machine-to-machine differences) a variance and/or errors in CD may occur in the manufacturing process. Generally, the actual pitch dependency of such errors depends on the specific layout of the pattern and the features, the aberration of the projection system of the lithographic apparatus in use, the properties of the radiation sensitive layer on the substrate, and the radiation beam properties such as illumination settings, and the exposure dose of radiation energy. Therefore, given a pattern to be provided by a patterning device and to be printed using a specific lithographic projection apparatus including a specific radiation source, one can identify data relating to iso-dense bias which are characteristic for that process, when executed on that lithographic system. In a situation where different lithographic projection apparatus (of the same type and/or of different types) are to be used for the same lithographic manufacturing process step, there is a problem of mutually matching the corresponding different iso-dense bias characteristics, such as to reduce CD variations occurring in the manufacturing process.
A known technique to match an iso-dense bias characteristic of a machine (for a process whereby an annular illumination mode is used) to an iso-dense bias characteristic of another machine is to change the σ-outer and σ-inner settings, while maintaining the difference between the σ-outer and σ-inner settings (i.e., whilst maintaining the annular ring width of the illumination mode) of one of the two machines. The nominal σ-settings are chosen so as to optimize the process latitude (in particular, the depth of focus and the exposure latitude). Therefore, this approach has the disadvantage that for the machine whereby the σ-settings are changed, the process latitude is becoming smaller and may become too small for practical use.
U.S. Patent Publication No. 2002/0048288A1 (CYMER) relates to an integrated circuit lithographic technique for controlling bandwidths wherein the laser beam bandwidth is controlled to produce an effective beam spectrum having at least two spectral peaks in order to produce improved pattern resolution in photo-resist film. U.S. Patent Publication No. 2002/0048288A1 is incorporated herein by reference.
U.S. Pat. No. 5,303,002 (INTEL) relates to a method and apparatus for patterning a photo-resist layer wherein a plurality of bands of radiation are used to provide an enhanced depth of focus. U.S. Pat. No. 5,303,002 is incorporated herein by reference.
The present inventors have identified the following. The finite size of a laser bandwidth introduces a smear out of the image of a feature over a focus range around the best focus position in a resist layer (dF/dλ=C, where F=Focus, λ=wavelength, and C=a constant. In other words, when, for example, a drawing shows an axis in “Focus (μm),” this could be replaced by “wavelength (pm).” This has an effect on image contrast at the wafer level; a large laser bandwidth introduces a lower image contrast at wafer level, but at the same time the focus performance with respect to maximum Depth-of-Focus (DOF) is improved. In general the useable range of an Exposure Dose (ED)—window is asymmetric around best focus. For example, contact holes tend to have an asymmetric useable range in defocus so closing earlier in one defocus direction, as compared to the other.
An aspect of at least one embodiment of the at least one aspect of the present invention seeks to effectively remove the asymmetry of the ED-window, thereby enabling a more predictable behaviour of printing integrity. This is particularly useful for wafers having a considerable topography.